|Arsenic Removal From Drinking Water by Adsorptive Media, EPA Demonstration Project at Golden Hills Community Services District in Tehachapi, CA, Final Performance Evaluation Report (EPA/600/R-10/011) March 2010
This report documents the activities performed and the results obtained for the arsenic (As) removal treatment technology demonstration project at Golden Hills Community Services District (GHCSD) located in Tehachapi, California. The objectives of the project were to evaluate the:
The project also characterized water in the distribution system and residuals generated by the treatment process. The types of data collected included system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M costs.
The Isolux arsenic treatment system consisted of two adsorption modules arranged in parallel, capable of treating up to 150 gallons per minute (gpm) of flow. Each module, designed for 75 gpm, consisted of a booster pump, a 1-micrometer bag filter, and two 20-inch by 48-inch carbon-steel filtration vessels, each containing nine Isolux-302M media cartridges. Each media cartridge was 4.55 inches in diameter and 42.25 inches in length and contained 0.32 cubic feet of Isolux-302M, a hydrous zirconium oxide media with amphoteric properties.
During the performance evaluation study from October 26, 2005, through March 20, 2007, three media runs were performed, each operating for a total run time of 1,377, 1,900, and 1,422 hours (or 21.9, 20.2, and 16.7 hours per day). Average flowrates for the runs were 79, 74, and 85 gpm. Based on the average flowrates, the empty bed contact times ranged from 0.9 to 1.2 minutes, compared with the design value of 0.5 minutes.
Among the 13 active wells at GHCSD, only Well C had elevated arsenic concentrations, which averaged 12.2 μg/L and existed primarily as soluble As(V). The pH values of raw water ranged from 7.4 to 7.9 and averaged 7.6, which is much lower than the zero point of charge for zirconium hydroxide (i.e., 10 to 11).
During Media Run 1, the system treated approximately 61,600 bed volumes (BV) of water before reaching 10 μg/L arsenic breakthrough. This run length was 41 percent lower than the vendor’s estimated 105,000 BV. An excessive amount of sediment was observed in the well water, necessitating frequent replacement of bag filters prior to the adsorption modules. It was possible that particles passed through the bag filters that blocked (or partially blocked) some passages on the media cartridges’ outer membrane, causing preferential flow and the short run length observed. Examination of the well revealed rusty areas on the drop-pipe, which prompted a decision by GHCSD to rehabilitate the well.
Following the well rehabilitation and media cartridge changeout, Media Run 2 began on April 27, 2006. The system treated 92,800 BV of water before reaching 10 μg/L arsenic breakthrough. Since Media Runs 1 and 2 operated under similar conditions, the well rehabilitation might have, in fact, contributed to the more extended media life observed.
Following media cartridge changeout, Media Run 3 began on August 17, 2006, and ended on March 20, 2007, with the system operating intermittently due to a lower demand in the winter. The system treated approximately 85,100 BV after reaching 10 μg/L arsenic breakthrough. Similar run lengths were observed during Media Runs 2 and 3. The intermittent system operation (i.e., 16.7 versus 20.2 hours per day) did not seem to affect the media run length.
The treatment system did not require backwash; therefore, spent media cartridges were the only residue generated. Spent Isolux-302M media passed Toxicity Characteristic Leaching Procedure tests and, therefore, could be disposed of as nonhazardous waste. However, MEI opted to send the spent media for beneficial reuse.
Comparison of the distribution system sampling results before and after system startup showed a slight decrease in the average arsenic concentration at each of the three sampling locations (i.e., from 2.8, 6.0, and 5.2 μg/L to 2.0, 3.3, and 3.1 μg/L, respectively). Most of the time, arsenic concentrations were much lower than those of the treatment effluent, presumably due to blending of the treated water with untreated water from wells where arsenic levels were not of concern. Lead and copper concentrations at the three sampling locations did not appear to be significantly impacted by the arsenic treatment system.
The capital investment cost was $76,840 and included $58,500 for equipment, $8,500 for engineering, and $9,840 for installation. Using the system’s rated capacity of 150 gpm, the capital cost was $512 per gpm (or $0.36 per gallons per day).
The O&M cost for the Isolux system included cost for media cartridge replacement and labor for routine operation. Based on the volumes processed during each media run prior to 10 μg/L arsenic breakthrough, the total O&M cost, including media cartridge replacement for Media Runs 1, 2, and 3, was $1.35, $0.89, and $0.98 per 1,000 gallons, respectively. Routine activities to operate and maintain the system consumed only 2.5 hours per week. Therefore, the estimated labor cost was $0.14 per 1,000 gallons of water treated, assuming that the system operates at 79.3 gpm for 19.6 hours per day and 7 days per week to produce 653,000 gallons of water per week.
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